Arachnid Fossils

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fossil arachnids

Fossil Arachnids are quite rare due to their delicate tissues.

Arachnida is one of the major arthropod groups. It includes spiders (Araneae), scorpions (Scorpiones), mites (Acari) and harvestmen (Opiliones), as well as a number of rarer and less familiar groups (Fig 1). The name Arachnida was introduced by the French zoologist Jean-Baptise Lamarck and is derived from Greek mythology: in one story, the maiden Arachne challenged the goddess Athene to a weaving contest, and was subsequently transformed into a spider — condemned to weave for evermore. There are about 100,000 living species of arachnids, with mites and spiders representing the most diverse and species-rich groups. Fossil arachnids are considerably rarer, with more than 1,700 described species (well over half of which are spiders) and a record that extends back to the Silurian Period. These oldest arachnids were probably among the first animals to move from water onto land.

Arachnids are instantly recognizable by their eight legs, although it should be mentioned that both mites and the rare order Ricinulei hatch as six-legged larvae, and acquire the last pair of legs later in development. The eight legs of arachnids are in fact part of a wider body plan in which the front half of the body — the prosoma or cephalothorax — bears six pairs of limbs. These are the chelicerae (or mouthparts), the pedipalps and four pairs of walking legs. This division of the body into a prosoma at the front and an opisthosoma (or abdomen) at the back is another textbook character of arachnids (Fig 2). It is very obvious in animals such as spiders, but in groups such as harvestmen the two halves of the body may be broadly joined together. Mites are also problematic, in that their body plan does not always conform to a neat prosoma/opisthosoma division.

The oldest arachnids of which we can be certain are found in the Silurian Period. There is a report of a mite from the earlier Ordovician Period, but this is controversial, because it is thought to belong to a highly evolved group. There is nothing in the fossil record that we could confidently call a ‘proto-arachnid’;  the oldest arachnids are scorpions, found in a recognizable form from the mid-Silurian onwards (Fig 5). There is some controversy about whether scorpions were terrestrial or aquatic at this time. In the late Silurian we see the first example of the trigonotarbid arachnids: a group that almost certainly did live on land.